Many types of analog and digital electrical equipment produce stray electromagnetic radiation, referred to as electromagnetic interference (EMI), which is emitted into the surrounding environment, The EMI usually results from analog circuit: components which oscillate at high frequencies, or from digital components which operate at high clocking or switching rates. These emissions are undesirable since they can, if sufficiently strong, interfere with the operation of radio receivers and other nearby electrical equipment. Regulations have been established for the maximum permissible EMI emissions from various types of electrical equipment, and these regulations must be taken into account when designing new equipment in which EMI might be a problem.
For some types of electronic equipment., such as telephone channel banks used to carry out analog-to-digital and digital-to-analog conversion between subscriber lines and telephone company lines, EMI reduction is difficult because of the basic design of the equipment and the practical need to allow access to the equipment by service personnel. Typically, telephone channel banks consist of rows of pull-out circuit cards, referred to as channel units, which are contained in a metal frame or housing. The housing is grounded and provides effective EMI shielding at the top, bottom, sides and back of the channel banks, but there is little shielding at the front since the frame must be left open to allow for the removal and replacement of the individual channel units. In essence, the rectangular front opening of each row or shelf in the housing serves as a slot or waveguide antenna for the electromagnetic radiation. Even though these openings are physically closed off by the channel unit faceplates when all of the channel units are fully inserted, they are electrically open since the channel unit faceplates are typically made of plastic and are essentially transparent to electromagnetic radiation.
Two approaches have been developed for reducing EMI emissions in telephone channel banks. In one approach, exemplified by U.S. Pat. Nos. 4,991,062 and 5,084,802, both to Nguyenngoc, a resilient metal strip is fastened to the back of the channel unit faceplate and is bent outwardly near its upper and lower ends to form spring contacts which extend rearwardly at an angle from the plane of the faceplate. When the channel unit is inserted into the grounded channel bank housing, the spring contacts are brought into contact with conductive areas of the housing located immediately above and below the channel bank shelf opening, thereby creating an electroconductive shunt across the shelf opening and reducing its effectiveness as a slot antenna. Although this arrangement is effective in theory, there is a practical disadvantage in that the spring contacts must essentially be flattened against the back surface of the faceplate during insertion of the channel unit in order to insure full contact with the conductive surfaces of the housing. If this does not occur, either because of poor manufacturing tolerances or because the channel unit has not been fully inserted, the spring contacts will touch the housing over a much smaller area. This can increase the impedance of the shunt (particularly if dirt or other contaminants are present) and reduce its ability to suppress EMI emissions. The degree of bending required to flatten the spring contacts can also deform the spring contacts over time, reducing their resiliency.
In the aforementioned copending U.S. patent application of Wilfred L. Gleadall, which is assigned to the assignee of the present invention, a different approach is taken. Rather than providing spring contacts at the top and bottom of the conductive strip for connecting both ends of the strip to the frame or housing, a spring contact is provided only at the top of the conductive strip. However, the conductive strip is also connected to ground by means of a printed ground wire on the channel unit circuit board itself, thereby allowing a separate ground connection to be made through the edge connector at the rear of the channel unit card. In this way, the conductive strip is reliably connected to ground without depending entirely on the quality of the electrical contact between the spring contact and the conductive surface of the housing. However, EMI suppression is affected to some extent by the lack of a direct shunt across the channel bank opening itself, and the need for a printed ground wire on the channel unit circuit board may be undesirable in some instances.